Transfer soup to a bowl and chill, covered, in a larger bowl of ice and cold water in the refrigerator for at least 8 hours.

Just before serving, discard cheesecloth bag and add grated zest and grapes to soup.

Cooks' notes:
· Soup (without grated zest and grapes) can be chilled up to 12 hours. Add zest and grapes just before serving.
· A metal spoon is the correct tool for stirring gelatin; a wooden one might transfer oils, and a whisk might incorporate tiny bubbles.